As a method of detecting an abnormality in a clock signal, there has been conventionally known a method using a watchdog timer. However, while being able to detect an increase in a clock frequency or a stoppage of a clock signal, the watchdog timer cannot detect a decrease in a clock frequency.
Further, JP 8-119553 A discloses a method of detecting an abnormality by dividing two clock signals with different frequencies, using them as reset signals for their respective clock counter circuits, and by counting the numbers of their respective clock pulse edges. In this method, however, since signals for determining a count-up period are generated through clock division, a circuit for this purpose needs to be added, which makes the circuit arrangement complicated and increases the failure rate. Besides, it is impossible to confirm whether or not the added circuit functions normally, which means a lack of reliability.